The Kaplan Daguerreotype of Abraham Lincoln is purported to be the earliest known photo of the 16th President, taken in the early 1840s when he was in his early 30s. The young man in the photo doesn't bare an obvious resemblance to a photo taken of Lincoln a few years later but the forensic evidence is compelling.
Numerous accounts have revealed that Lincoln underwent a noticeable change in his physical appearance beginning in January 1841 as a result of a grave emotional crisis. This coincides with his reported failure to go through with his scheduled marriage to Mary Todd, leaving her literally waiting for him at the altar. (They were married the following year.) This emotional crisis, just one of a series of such episodes to plague him throughout his life, was the cause of Lincoln losing a considerable amount of weight.